The present invention relates to a device for protecting a low power semiconductor circuit against overvoltages.
For many applications in the field of electronics, it is desirable to employ low power semiconductor circuits. Accordingly, there is a need for an efficient, inexpensive and reliable overvoltage protection device that will protect such circuits against overvoltages on their input leads. Typical semiconductor components for such low power circuits are: diodes, transistors, integrated circuits, and other components designed for maximum currents of one or a few amperes and maximum voltages of a hundred or a few hundred volts. Overvoltage protection devices may be used to protect the semiconductor circuitry that is employed, for example, in telephone stations and in telephone sets, or, in general, for semiconductor signal processing circuits.
It is known to use gas discharge tubes to provide overvoltage protection. However, a disadvantage of such tubes is that the tubes have a slow ignition time, thereby allowing an overvoltage to reach a relatively large amplitude before the gas discharge tube is activated. In addition, the ignition voltage cannot be precisely set in gas discharge tubes.
Zinc oxide varistors have also been used as overvoltage protection devices. However, such varistors have a relatively high leakage current during normal operation and also provide a poorly defined level of protection. In addition, the varistors show an appreciable degradation in response to repeated overvoltages.
It is also known to use zener diodes as overvoltage protection devices for semiconductor components. However, zener diodes must be provided with relatively large operational dimensions in order to provide a sufficient capacity for energy absorption. Moreover, both varistors and zener diodes show a relatively high capacitance, which has a harmful influence on the transmission of high frequency signals on the lines to which an overvoltage protection device is connected.
Accordingly, it is a primary object of the invention to provide a fast, efficient, inexpensive and reliable overvoltage protection device having relatively small operational dimensions and providing a well-defined and constant level of protection.
Another object of the invention is to provide such a device that has a low leakage current and a low capacitance.
These and other objects of this invention will become apparent from a review of the detailed specification which follows and a consideration of the accompanying drawings.